1. Field of the Invention
This invention relates to a method for fabricating ferroelectric polymer film, preferably usable for a piezoelectric element, a pyroelectric element, an opto-electrical element, and a non-linear optical element.
2. Description of the Prior Art
A film made of a ferroelectric polymer has larger flexibility than a film made of a ferroelectric inorganic material, and has a large piezoelectric effect induced by poling treatment. Therefore, the ferroelectric polymer film may be used as material for various piezoelectric devices, and particularly, employed aggressively for piezoelectric sensors and transducers because of the low acoustic impedance.
As a ferroelectric polymer film with a large electro-mechanical conversion efficiency, a copolymer of vinylidene fluoride and trifluoroethylene or tetrafluoroethylene is well known. The ferroelectric copolymer film is composed of the aggregation of plural lamellar crystal portions, where the mechanical properties and breaking strength are determined on the cohesive forces between the adjacent crystal portions. Therefore, the ferroelectric copolymer film is fragile and can not exhibit flexibility peculiar to a polymer.
In this point of view, the inventor has conceived to fabricate a uniaxially drawn film made of a copolymer of vinylidene fluoride and trifluoroethylene and to thermally treat and thus, crystallize the uniaxially drawn film with keeping the surfaces of the uniaxially drawn film in free surface condition, thereby to obtain a ferroelectric single crystalline polymer film (Japanese Patent No. 2681032). In this case, the molecular chains of such a ferroelectric single crystalline polymer film are oriented in the drawing direction at high degree, and also, the crystal axes (a axis and b axis) perpendicular to the drawing direction are oriented in given directions within the plane perpendicular to the drawing direction. Moreover, in the ferroelectric single crystalline polymer film, extended chain crystal portions are created and thus, the lamellar crystal portions are vanished.
As a result, the longitudinal wave velocity and the Young's modulus of the ferroelectric single crystalline polymer film can be improved up to 7930 m/sec and 121 GPa at a temperature of 10K (−263° C.) and up to 3800 m/sec and 27.9 GPa at a temperature of 300K (27° C.), respectively. The above physical properties are much larger than a lamellar crystalline polymer film.
However, the Young's modulus of the ferroelectric single crystalline copolymer film of vinylidene fluoride and trifluoroethylene is theoretically estimated 230 GPa, and thus, the current Young's modulus as mentioned above is extremely smaller than the theoretical value. In a sensor or transducer utilizing piezoelectric effect, the electro-mechanical conversion efficiency becomes larger as the young's modulus becomes larger. Therefore, it is desired to enhance the Young's modulus of such a ferroelectric single crystalline copolymer film.